1. Field of the Invention
The present invention relates to a semiconductor/electrode contact structure, and a semiconductor device using the same. Examples of the semiconductor device include a photoelectric conversion device such as a solar cell device, a diode, a transistor, and a thyristor.
2. Description of the Related Art
A semiconductor device has a contact between an electrode composed of a metal or a conductive material such as a conductive oxide material and a semiconductor. On the side of the semiconductor in the semiconductor/electrode contact, a defect level called an interface level is generally produced. Such a defect level occurs due to impurities and distortion, non-equilibrium in a semiconductor forming process, etc., and functions as a carrier recombination center. Therefore, the semiconductor/electrode contact greatly affects semiconductor device characteristics.
The recombination of carries in the semiconductor/electrode contact interface generally functions as a factor which degrades current/voltage characteristics of the semiconductor device. A recombination rate R in the interface is proportional to a recombination center density Nr in the interface and a minority carrier density n on an interface surface.R∝Nr×n
It is considered that the recombination center density Nr and a defect level density Nd in the interface are exactly in a relationship of Nr<Nd. In a range handled in the present case, however, there is hardly any problem even if it is considered that Nr≈Nd.
The variation in carrier recombination characteristics in the interface greatly affects the yield of the semiconductor device. In order to improve the characteristics of the semiconductor device and to improve the yield thereof, therefore, it is important to sufficiently reduce a carrier recombination rate R in the semiconductor/electrode contact interface.
A solar cell which is represented by a photoelectric conversion device will be described as an example.
Generally, the solar cell has a pn junction or a pin junction composed of a semiconductor. Examples of the electrode are an electrode coming into contact with a p-type semiconductor and an electrode coming into contact with an n-type semiconductor.
FIG. 10 illustrates an example of a solar cell device structure in a case where a pin junction is used. In FIG. 10, reference numeral 1 denotes a substrate, reference numeral 2 denotes a front electrode, reference numeral 3 denotes a semiconductor multi-layer film, reference numeral 3a denotes a p-type semiconductor layer, reference numeral 3b denotes an i-type semiconductor layer, reference numeral 3c denotes an n-type semiconductor layer, and reference numeral 4 denotes a rear electrode.
Here, light is incident on the substrate 1, and is absorbed and photoelectrically converted in the semiconductor multi-layer film 3, to generate an electron-hole pair. In the case of the pin junction as in the illustrative example, however, the electron-hole pair generated by absorbing and photoelectrically converting light particularly in the i-type semiconductor layer 3b serving as a photoactive layer is a main origin of photovoltaic power. The generated electrons and holes (photo-generated carriers) are respectively swept toward the n-type semiconductor layer 3c or the p-type semiconductor layer 3a in accordance with an internal electric field formed in the i-type semiconductor layer 3b, so that the electrons are excess in the n-type semiconductor layer 3c, and the holes are excess in the p-type semiconductor layer 3a. Accordingly, a forward bias voltage is generated with respect to the pin junction. The excess carriers respectively flow into the electrodes, so that a current flows through a circuit which connects a solar cell and a load to each other. The product of the forward bias voltage and the current is photovoltaic power to be outputted.
FIG. 11 is a band diagram of the solar cell. In FIG. 11, Ec indicates an energy position in a lower part of a conduction band edge, Ev indicates an energy position in an upper part of a valence band edge, Ed indicates a defect level (an interface defect level) on the side of the semiconductor in the semiconductor/electrode interface, and Ef indicates a Fermi level.
Here, the existence of the defect level in the semiconductor/electrode interface (in the illustrative example, the first electrode 2/the p-type semiconductor layer 3a, and the n-type semiconductor layer 3c/the second electrode 4) can adversely affect solar cell characteristics mainly due to the following two factors.
The first factor is degradation (increase) in diode characteristics (in the case of the solar cell, increase in a so-called dark current). This is caused by the production of a recombination current in the interface. The increase in the dark current causes reduction in an open circuit voltage Voc and a fill factor FF as the solar cell characteristics.
The second factor is reduction in a light current. This is caused by the fact that the photo-generated carriers generated in the semiconductor region are lost upon being recombined in the interface and causes reduction in a short-circuit current density Jsc as the solar cell characteristics.
There is a measure to highly dope a portion of a semiconductor interface region in the semiconductor/electrode contact against the problems. This is a method utilizing the fact that a recombination rate R in an interface is reduced because a minority carrier density n is generally reduced in a highly doped portion, and is generally called a HL (High-Low) junction. The technique may, in some cases, be called a BSF structure in connection with the historical process of an effect (BSF effect=Back Surface Field effect) particularly in a case where it is formed on the back surface region of the solar cell.
The measure allows the minority carrier density n to be reduced. However, there is no reason that the recombination center density Nr caused by the interface defect level is expected to decrease. Contrary to this, as a doping amount increases, a distortion amount increases, resulting in defect formation. Consequently, the quality of the semiconductor is generally degraded. Therefore, it is considered that the recombination center density Nr in the interface is generally increased.
It is herein considered that the maximum value of the interface defect level density is given as a DB density in a case where all coupling hands in the interface becomes a dangling bond (DB). In the case of Si, for example, the DB density in this case is a value which is as large as approximately 1×1015/cm2, and the recombination center density Nr is also a very large value on approximately the same order.
As described above, the recombination rate R in the interface is proportional to the product of the minority carrier density n and the recombination center density Nr. Even if the minority carrier density n can be reduced, therefore, the recombination rate R cannot be sufficiently reduced unless the recombination center density Nr can be reduced (or if the recombination center density Nr is conversely increased).
In practice, it cannot be said that the reduction in the recombination rate R in the interface in the measure is not sufficient to further increase the efficiency of the solar cell. Therefore, a technique for further interposing a very thin SiO2 film between the HL junction and the electrode, for example, to also reduce the recombination center density Nr is also developed. In the improving technique, however, it is not easy to form a high-quality SiO2 film for sufficiently reducing the recombination center density Nr into a very thin film. Further, a high-temperature process is required to form the high-quality SiO2 film. In the actual application, therefore, various restrictions in processes occur even in a bulk-type silicon solar cell. The method cannot be applied at all to the thin film silicon solar cell manufactured in low-temperature processes.
Furthermore, there is a method of widening a band gap in a portion, of an interface region, on the side of the semiconductor in the semiconductor/electrode contact. In the portion where the gap is widened, the minority carrier density n is generally reduced, so that the recombination rate R in the interface is reduced.
From a historical view, an example of an amorphous silicon solar cell using hydrogenated amorphous silicon (a-Si:H), in which a p-type layer in contact with a front electrode (a transparent conductive film such as SnO2) is composed of a hydrogenated amorphous silicon carbide (a-SiC:H, a band gap≈2.0 eV), and a hetero junction is formed between the p-type layer and an I-type layer composed of an a-Si:H (a band gap≈1.8 eV) serving as a photoactive layer, has been well known.
Also in this method, it is possible to reduce the minority carrier density n. However, it is not assured that the defect level density Nr in the interface is not increased. Generally, the widening of a gap not only reduces a minority carrier density n but also reduces majority carriers. If the gap is too widened, therefore, the flow of the majority carriers is prevented, so that ohmic characteristics between the semiconductor and the electrode, which are required as a premise, are degraded. Therefore, the widening of the gap has a limitation.
Although description was made by taking as an example the solar cell which is represented by the photoelectric conversion device out of the semiconductor devices, the nature in principle of the above-mentioned problem (the recombination of carriers by the existence of the interface defect level) is generally common to the semiconductor devices in general each having the semiconductor/electrode contact. In a diode, for example, if the recombination rate R in the semiconductor/electrode contact is large, this causes a factor which degrades current-voltage characteristics such as rise characteristics by increase in a dark current. Further, in a transistor, this causes a factor which degrades current/voltage characteristics such as ON/OFF characteristics.
Furthermore, it is known that it is very effective to stack a plurality of semiconductor junction layers in increasing the efficiency of the solar cell, for example. As a measure to achieve higher efficiency, however, a technique for introducing a transparent intermediate layer between semiconductor junction layers has been recently proposed (see JP02-76266, A (1990) and JP02-76267, A (1990)).
This serves to adjust spectrum distribution (distribution dependent on a wavelength) of incident light energy among the junction layers by the existence of the transparent intermediate layer to perform more efficient photoelectric conversion.
Here, the function of the transparent intermediate layer will be simply described using FIG. 12. FIG. 12 is a diagram showing the configuration of a conventional thin film solar cell of a super-straight type in which light is incident on a substrate, having two semiconductor junction layers 31 and 32, and obtained by introducing a transparent intermediate layer 5 between the semiconductor junction layers 31 and 32 (JP04-127580, A (1992)). FIG. 13 is a band diagram of a pin-type semiconductor device in the thin film solar cell shown in FIG. 12.
On a transparent substrate 1, a front electrode 2 composed of a transparent conductive material, a semiconductor multi-layer film 3, and a rear electrode 4 are successively formed. Further, the semiconductor multi-layer film 3 is formed with a conductive transparent intermediate layer 5 having conductive properties interposed between the first semiconductor junction layers 31 and the second semiconductor junction layer 32. The semiconductor junction layer is generally composed of a photoelectric conversion cell having a pin junction. Here, the first semiconductor junction layer 31 is composed of a pin junction of a p-type layer 31a, a photoactive layer 31b of an i type, and an n-type layer 31c, and the second semiconductor junction layer 32 is composed of a pin junction of a p-type layer 32a, a photoactive layer 32b of an i type, and an n-type layer 32c. 
From the viewpoint of high efficiency, a material having a large band gap energy, for example, an amorphous silicon material represented by hydrogenated amorphous silicon is generally used for the photoactive layer 31b in the first semiconductor junction layer 31 serving as a top cell positioned on the side of a light incidence surface out of the semiconductor junction layers. On the other hand, a material having small band gap energy, for example, a microcrystalline silicon or a nanocrystalline silicon is used for the photoactive layer 32b in the second semiconductor junction layer 32 serving as a bottom cell positioned on the opposite side of the light incidence surface.
If light (hν) is incident on the transparent substrate 1, the light passes through the front electrode 2, and is photoelectrically converted in the first semiconductor junction layer 31 and the second semiconductor junction layer 32, producing photovoltaic power.
Particularly, the first semiconductor junction layer 31 has high light absorption characteristics for short-wavelength light because the photoactive layer 31b includes an amorphous silicon material having large band gap energy. On the other hand, the second semiconductor junction layer 32 has high, light absorption characteristics for long-wavelength light because the photoactive layer 32b includes a microcrystalline silicon or a nanocrystalline silicon having a small band gap energy. Therefore, it is possible to perform photoelectric conversion over a wide wavelength range of incident light.
The refractive index and the film thickness of the transparent intermediate layer 5 provided between the first semiconductor junction layer 31 and the second semiconductor junction layer 32 are adjusted, thereby making it possible to more easily reflect a short wavelength component which cannot be absorbed entirely in the photoactive layer 31b in the first semiconductor junction layer 31 and more easily transmit a long wavelength component which is absorbed in the photoactive layer 32b in the second semiconductor junction layer 32 in light which has reached the transparent intermediate layer 5 after being incident from a light incidence surface to pass through the first semiconductor junction layer 31.
This allows the light-energy density of the short wavelength component to be raised in the semiconductor junction layer 31 positioned on the side of the light incidence surface with respect to the transparent intermediate layer 5, while allowing the light energy density of the long wavelength component to be raised in the semiconductor junction layer 32 positioned on the downstream side in light transmission of the transparent intermediate layer 5.
It is possible to distribute light energy corresponding to band gap energy, that is, distribute light energy in consideration of a wavelength by disposing the transparent intermediate layer 5, as described above. Therefore, efficient photoelectric conversion can be performed, thereby allowing a multi-junction type thin film solar cell device to be made highly efficient.